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Tuesday, December 13, 2011

Thorium nuclear reactor

Thorium occurs in several minerals, the most common being the rare earth-thorium-phosphate mineral, monazite, which contains up to about 12% thorium oxide, but average 6-7%…
In India, both Kakrapar-1 and -2 units are loaded with 500 kg of thorium fuel in order to improve their operation when newly-started. Kakrapar-1 was the first reactor in the world to use thorium, rather than depleted uranium, to achieve power flattening across the reactor core. In 1995, Kakrapar-1 achieved about 300 days of full power operation and Kakrapar-2 about 100 days utilising thorium fuel. The use of thorium-based fuel was planned in Kaiga-1 and -2 and Rajasthan-3 and -4 (Rawatbhata) reactors.
With about six times more thorium than uranium, India has made utilisation of thorium for large-scale energy production a major goal in its nuclear power program, utilising a three-stage concept:
· Pressurised Heavy Water Reactors (PHWRs, elsewhere known as CANDUs) fuelled by natural uranium, plus light water reactors, produce plutonium.
· Fast Breeder Reactors (FBRs) use this plutonium-based fuel to breed U-233 from thorium. The blanket around the core will have uranium as well as thorium, so that further plutonium (ideally high-fissile Pu) is produced as well as the U-233. Then
· Advanced Heavy Water Reactors burn the U-233 and this plutonium with thorium, getting about 75% of their power from the thorium.
The spent fuel will then be reprocessed to recover fissile materials for recycling.
This Indian program has moved from aiming to be sustained simply with thorium to one “driven” with the addition of further fissile uranium and plutonium, to give greater efficiency.
UIC Briefing Paper # 67
May 2007

· Thorium is much more abundant in nature than uranium.
· Thorium can also be used as a nuclear fuel through breeding to uranium-233 (U-233).
· When this thorium fuel cycle is used, much less plutonium and other transuranic elements are produced, compared with uranium fuel cycles.
· Several reactor concepts based on thorium fuel cycles are under consideration.

Thorium is a naturally-occurring, slightly radioactive metal discovered in 1828 by the Swedish chemist Jons Jakob Berzelius, who named it after Thor, the Norse god of thunder. It is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium. Soil commonly contains an average of around 6 parts per million (ppm) of thorium.
Thorium occurs in several minerals, the most common being the rare earth-thorium-phosphate mineral, monazite, which contains up to about 12% thorium oxide, but average 6-7%. There are substantial deposits in several countries (see table). Thorium-232 decays very slowly (its half-life is about three times the age of the earth) but other thorium isotopes occur in its and in uranium’s decay chains. Most of these are short-lived and hence much more radioactive than Th-232, though on a mass basis they are negligible.
World thorium resources
(economically extractable):
Reserves (tonnes)
300 000
290 000
170 000
160 000
100 000
South Africa
35 000
16 000
Other countries
95 000
World total
1 200 000
source: US Geological Survey, Mineral Commodity Summaries, January 1999 The 2005 IAEA-NEA “Red Book” gives a figure of 4.5 million tonnes of reserves and additional resources, but points out that this excludes data from much of the world. Geoscience Australia confirms the above 300,000 tonne figure for Australia, but stresses that this is based on assumptions, not direct geological data in the same way as most mineral rsources.
When pure, thorium is a silvery white metal that retains its lustre for several months. However, when it is contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and eventually black. Thorium oxide (ThO2), also called thoria, has one of the highest melting points of all oxides (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light. Because of these properties, thorium has found applications in light bulb elements, lantern mantles, arc-light lamps, welding electrodes and heat-resistant ceramics. Glass containing thorium oxide has a high refractive index and dispersion and is used in high quality lenses for cameras and scientific instruments.
Thorium as a nuclear fuel Thorium, as well as uranium, can be used as a nuclear fuel. Although not fissile itself, thorium-232 (Th-232) will absorb slow neutrons to produce uranium-233 (U-233), which is fissile. Hence like uranium-238 (U-238) it is fertile.
In one significant respect U-233 is better than uranium-235 and plutonium-239, because of its higher neutron yield per neutron absorbed. Given a start with some other fissile material (U-235 or Pu-239), a breeding cycle similar to but more efficient than that with U-238 and plutonium (in slow-neutron reactors) can be set up. The Th-232 absorbs a neutron to become Th-233 which normally decays to protactinium-233 and then U-233. The irradiated fuel can then be unloaded from the reactor, the U-233 separated from the thorium, and fed back into another reactor as part of a closed fuel cycle.
Over the last 30 years there has been interest in utilising thorium as a nuclear fuel since it is more abundant in the Earth’s crust than uranium. Also, all of the mined thorium is potentially useable in a reactor, compared with the 0.7% of natural uranium, so some 40 times the amount of energy per unit mass might theoretically be available (withouit recourse to fast breeder reactors).
A major potential application for conventional PWRs involves fuel assemblies arranged so that a blanket of mainly thorium fuel rods surrounds a more-enriched seed element containing U-235 which supplies neutrons to the subcritical blanket. As U-233 is produced in the blanket it is burned there. This is the Light Water Breeder Reactor concept which was successfully demonstrated in the USA in the 1970s.
It is currently being developed in a more deliberately proliferation-resistant way. The central seed region of each fuel assembly will have uranium enriched to 20% U-235. The blanket will be thorium with some U-238, which means that any uranium chemically separated from it (for the U-233 ) is not useable for weapons. Spent blanket fuel also contains U-232, which decays rapidly and has very gamma-active daughters creating significant problems in handling the bred U-233 and hence conferring proliferation resistance. Plutonium produced in the seed will have a high proportion of Pu-238, generating a lot of heat and making it even more unsuitable for weapons than normal reactor-grade Pu.
A variation of this is the use of whole homogeneous assembles arranged so that a set of them makes up a seed and blanket arrangement. If the seed fuel is metal uranium alloy instead of oxide, there is better heat conduction to cope with its higher temperatures. Seed fuel remains three years in the reactor, blanket fuel for up to 14 years.
Since the early 1990s Russia has had a program to develop a thorium-uranium fuel, which more recently has moved to have a particular emphasis on utilisation of weapons-grade plutonium in a thorium-plutonium fuel.
The program is based at Moscow’s Kurchatov Institute and involves the US company Thorium Power and US government funding to design fuel for Russian VVER-1000 reactors. Whereas normal fuel uses enriched uranium oxide, the new design has a demountable centre portion and blanket arrangement, with the plutonium in the centre and the thorium (with uranium) around it*. The Th-232 becomes U-233, which is fissile – as is the core Pu-239. Blanket material remains in the reactor for 9 years but the centre portion is burned for only three years (as in a normal VVER). Design of the seed fuel rods in the centre portion draws on extensive experience of Russian navy reactors.
*More precisely: A normal VVER-1000 fuel assembly has 331 rods each 9 mm diameter forming a hexagonal assembly 235 mm wide. Here, the centre portion of each assembly is 155 mm across and holds the seed material consisting of metallic Pu-Zr alloy (Pu is about 10% of alloy, and isotopically over 90% Pu-239) as 108 twisted tricorn-section rods 12.75 mm across with Zr-1%Nb cladding. The sub-critical blanket consists of U-Th oxide fuel pellets (1:9 U:Th, the U enriched up to almost 20%) in 228 Zr-1%Nb cladding tubes 8.4 mm diameter – four layers around the centre portion. The blanket material achieves 100 GWd/t burn-up. Together as one fuel assembly the seed and blanket have the same geometry as a normal VVER-100 fuel assembly.
The thorium-plutonium fuel claims four advantages over MOX: proliferation resistance, compatibility with existing reactors – which will need minimal modification to be able to burn it, and the fuel can be made in existing plants in Russia. In addition, a lot more plutonium can be put into a single fuel assembly than with MOX, so that three times as much can be disposed of as when using MOX. The spent fuel amounts to about half the volume of MOX and is even less likely to allow recovery of weapons-useable material than spent MOX fuel, since less fissile plutonium remains in it. With an estimated 150 tonnes of weapons Pu in Russia, the thorium-plutonium project would not necessarily cut across existing plans to make MOX fuel.
In 2007 Thorium Power formed an alliance with Red Star nuclear design bureau in Russia which will take forward the program to demonstrate the technology in lead-test fuel assemblies in full-sized commercial reactors.
R&D history The use of thorium-based fuel cycles has been studied for about 30 years, but on a much smaller scale than uranium or uranium/plutonium cycles. Basic research and development has been conducted in Germany, India, Japan, Russia, the UK and the USA. Test reactor irradiation of thorium fuel to high burnups has also been conducted and several test reactors have either been partially or completely loaded with thorium-based fuel.
Noteworthy experiments involving thorium fuel include the following, the first three being high-temperature gas-cooled reactors:
· Between 1967 and 1988, the AVR experimental pebble bed reactor at Julich, Germany, operated for over 750 weeks at 15 MWe, about 95% of the time with thorium-based fuel. The fuel used consisted of about 100 000 billiard ball-sized fuel elements. Overall a total of 1360 kg of thorium was used, mixed with high-enriched uranium (HEU). Maximum burnups of 150,000 MWd/t were achieved.
· Thorium fuel elements with a 10:1 Th/U (HEU) ratio were irradiated in the 20 MWthDragon reactor at Winfrith, UK, for 741 full power days. Dragon was run as an OECD/Euratom cooperation project, involving Austria, Denmark, Sweden, Norway and Switzerland in addition to the UK, from 1964 to 1973. The Th/U fuel was used to ‘breed and feed’, so that the U-233 formed replaced the U-235 at about the same rate, and fuel could be left in the reactor for about six years.
· General Atomics’ Peach Bottom high-temperature, graphite-moderated, helium-cooled reactor (HTGR) in the USA operated between 1967 and 1974 at 110 MWth, using high-enriched uranium with thorium.
· In India, the Kamini 30 kWth experimental neutron-source research reactor using U-233, recovered from ThO2 fuel irradiated in another reactor, started up in 1996 near Kalpakkam. The reactor was built adjacent to the 40 MWt Fast Breeder Test Reactor, in which the ThO2 is irradiated.
· In the Netherlands, an aqueous homogenous suspension reactor has operated at 1MWth for three years. The HEU/Th fuel is circulated in solution and reprocessing occurs continuously to remove fission products, resulting in a high conversion rate to U-233.
· There have been several experiments with fast neutron reactors.
Power reactors
Much experience has been gained in thorium-based fuel in power reactors around the world, some using high-enriched uranium (HEU) as the main fuel:
· The 300 MWe THTR reactor in Germany was developed from the AVR and operated between 1983 and 1989 with 674,000 pebbles, over half containing Th/HEU fuel (the rest graphite moderator and some neutron absorbers). These were continuously recycled on load and on average the fuel passed six times through the core. Fuel fabrication was on an industrial scale.
· The Fort St Vrain reactor was the only commercial thorium-fuelled nuclear plant in the USA, also developed from the AVR in Germany, and operated 1976 – 1989. It was a high-temperature (700°C), graphite-moderated, helium-cooled reactor with a Th/HEU fuel designed to operate at 842 MWth (330 MWe). The fuel was in microspheres of thorium carbide and Th/U-235 carbide coated with silicon oxide and pyrolytic carbon to retain fission products. It was arranged in hexagonal columns (‘prisms’) rather than as pebbles. Almost 25 tonnes of thorium was used in fuel for the reactor, and this achieved 170,000 MWd/t burn-up.
· Thorium-based fuel for Pressurised Water Reactors (PWRs) was investigated at theShippingport reactor in the USA using both U-235 and plutonium as the initial fissile material. It was concluded that thorium would not significantly affect operating strategies or core margins. The light water breeder reactor (LWBR) concept was also successfully tested here from 1977 to 1982 with thorium and U-233 fuel clad with Zircaloy using the ‘seed/blanket’ concept.
· The 60 MWe Lingen Boiling Water Reactor (BWR) in Germany utilised Th/Pu-based fuel test elements.
In India, both Kakrapar-1 and -2 units are loaded with 500 kg of thorium fuel in order to improve their operation when newly-started. Kakrapar-1 was the first reactor in the world to use thorium, rather than depleted uranium, to achieve power flattening across the reactor core. In 1995, Kakrapar-1 achieved about 300 days of full power operation and Kakrapar-2 about 100 days utilising thorium fuel. The use of thorium-based fuel was planned in Kaiga-1 and -2 and Rajasthan-3 and -4 (Rawatbhata) reactors.
With about six times more thorium than uranium, India has made utilisation of thorium for large-scale energy production a major goal in its nuclear power program, utilising a three-stage concept:
· Pressurised Heavy Water Reactors (PHWRs, elsewhere known as CANDUs) fuelled by natural uranium, plus light water reactors, produce plutonium.
· Fast Breeder Reactors (FBRs) use this plutonium-based fuel to breed U-233 from thorium. The blanket around the core will have uranium as well as thorium, so that further plutonium (ideally high-fissile Pu) is produced as well as the U-233. Then
· Advanced Heavy Water Reactors burn the U-233 and this plutonium with thorium, getting about 75% of their power from the thorium.
The spent fuel will then be reprocessed to recover fissile materials for recycling.
This Indian program has moved from aiming to be sustained simply with thorium to one “driven” with the addition of further fissile uranium and plutonium, to give greater efficiency.
Another option for the third stage, while continuing with the PHWR and FBR programs, is the subcritical Accelerator-Driven Systems (ADS), – see below.
Emerging advanced reactor concepts Concepts for advanced reactors based on thorium-fuel cycles include:
· Light Water Reactors – With fuel based on plutonium oxide (PuO2), thorium oxide (ThO2) and/or uranium oxide (UO2) particles arranged in fuel rods.
· High-Temperature Gas-cooled Reactors (HTGR) of two kinds: pebble bed and with prismatic fuel elements.
Gas Turbine-Modular Helium Reactor (GT-MHR) – Research on HTGRs in the USA led to a concept using a prismatic fuel. The use of helium as a coolant at high temperature, and the relatively small power output per module (600 MWth), permit direct coupling of the MHR to a gas turbine (a Brayton cycle), resulting in generation at almost 50% thermal efficiency. The GT-MHR core can accommodate a wide range of fuel options, including HEU/Th, U-233/Th and Pu/Th. The use of HEU/Th fuel was demonstrated in the Fort St Vrain reactor (see above).
Pebble-Bed Modular reactor (PBMR) – Arising from German work the PBMR was conceived in South Africa and is now being developed by a multinational consortium. It can potentially use thorium in its fuel pebbles.
· Molten salt reactors – This is an advanced breeder concept, in which the fuel is circulated in molten salt, without any external coolant in the core. The primary circuit runs through a heat exchanger, which transfers the heat from fission to a secondary salt circuit for steam generation. It was studied in depth in the 1960s, but is now being revived because of the availability of advanced technology for the materials and components.
· Advanced Heavy Water Reactor (AHWR) – India is working on this, and like the Canadian CANDU-NG the 250 MWe design is light water cooled. The main part of the core is subcritical with Th/U-233 oxide, mixed so that the system is self-sustaining in U-233. A few seed regions with conventional MOX fuel will drive the reaction and give a negative void coefficient overall.
· CANDU-type reactors – AECL is researching the thorium fuel cycle application to enhanced CANDU-6 and ACR-1000 reactors. With 5% plutonium (reactor grade) plus thorium high burn-up and low power costs are indicated.
· Plutonium disposition – Today MOX (U,Pu) fuels are used in some conventional reactors, with Pu-239 providing the main fissile ingredient. An alternative is to use Th/Pu fuel, with plutonium being consumed and fissile U-233 bred. The remaining U-233 after separation could be used in a Th/U fuel cycle.
Use of thorium in Accelerator Driven Systems (ADS) In an ADS system, high-energy neutrons are produced through the spallation reaction of high-energy protons from an accelerator striking heavy target nuclei (lead, lead-bismuth or other material). These neutrons can be directed to a subcritical reactor containing thorium, where the neutrons breed U-233 and promote the fission of it. There is therefore the possibility of sustaining a fission reaction which can readily be turned off, and used either for power generation or destruction of actinides resulting from the U/Pu fuel cycle. The use of thorium instead of uranium means that less actinides are produced in the ADS itself. (see paper on Accelerator-Driven Nuclear Energy).
Developing a thorium-based fuel cycle Despite the thorium fuel cycle having a number of attractive features, development even on the scale of India’s has always run into difficulties. Problems include:
· the high cost of fuel fabrication, due partly to the high radioactivity of U-233 chemically separated from the irradiated thorium fuel. Separated U-233 is always contaminated with traces of U-232 (69 year half life but whose daughter products such as thallium-208 are strong gamma emitters with very short half lives);
· the similar problems in recycling thorium itself due to highly radioactive Th-228 (an alpha emitter with 2 year half life) present;
· some weapons proliferation risk of U-233 (if it could be separated on its own); and
· the technical problems (not yet satisfactorily solved) in reprocessing.
Much development work is still required before the thorium fuel cycle can be commercialised, and the effort required seems unlikely while (or where) abundant uranium is available. In this respect international moves to bring India into the ambit of international trade will be critical. If India has ready access to traded uranium and conventional reactor designs, it may not persist with the thorium cycle.
Nevertheless, the thorium fuel cycle, with its potential for breeding fuel without the need for fast-neutron reactors, holds considerable potential long-term. It is a significant factor in the long-term sustainability of nuclear energy.
Thorium based fuel options for the generation of electricity: Developments in the 1990s, IAEA-TECDOC-1155, International Atomic Energy Agency, May 2000.The role of thorium in nuclear energy, Energy Information Administration/Uranium Industry Annual, 1996, p.ix-xvii.Nuclear Chemical Engineering (2nd Ed.), Chapter 6: Thorium, M Benedict, T H Pigford and H W Levi, 1981, McGraw-Hill, p.283-317, ISBN: 0-07-004531-3.
See also: lead paper in Indian Nuclear Society 2001 conference proceedings, vol 2.
Kazimi M.S. 2003, Thorium Fuel for Nuclear Energy,
American Scientist Sept-Oct 2003.
Morozov et al 2005, Thorium fuel as a superior approach to disposing of excess weapons-grade plutonium in Russian VVER-1000 reactors.
Nuclear Future?
OECD NEA & IAEA, 2006,
Uranium 2005: Resources, Production and Demand

Transcript, links and further information for ‘Thorium Reactors’Narration Energy: it’s something you normally don’t think about. It pours in from the burning of coal, oil and gas, or from hydro-electric dams. And we keep needing more and more of it — but that means more greenhouse gases. Nuclear power doesn’t generate any gases — but you try and say that in polite company. Wilson da Silva PTC It’s not every day you hear about a potential solution to the energy problems of the 21st century in a cafe. But I did — from my friend Andrew. Dr Andrew Studer, Physicist Heard about a great new idea the other day; a thing called an energy amplifier. It’s like a nuclear reactor driven by a particle accelerator. And the whole point is you can use thorium instead of uranium, and apparently this produces a heap less waste. The thing can never melt down or blow up. And you can actually use it to reprocess plutonium and nuclear waste from old bombs. Wilson da Silva Are we talking about a green nuclear reactor here? Dr Andrew Studer, Physicist Well, the whole thing is that it uses thorium which you can’t do in an ordinary reactor. You don’t have to have this particle accelerator driving it to make it work. Wilson da Silva And, what, you can turn it off if there’s a risk of a meltdown? Dr Andrew Studer, Physicist Well, it can’t meltdown because you are in complete control of how much energy’s going into it in the first place. So there’s no way the thing can ever overheat and blow up.Narration It sounds too good to be true, doesn’t it? But there’s a whole community of scientists out there working on this rather novel idea of a thorium reactor, otherwise known as an energy amplifier … calculating, designing and experimenting. Three prototype reactors are to be built in Spain, and more are on the drawing boards. Wilson da Silva PTC It’s sort of like a regular reactor, only it uses thorium instead. You know what we really need? We need to see how a regular nuclear reactor works. But it’s not like we have that many of them in Australia. Wilson da Silva PTC I’m with Dr Sue Town who’s a physicist here at the HIFAR reactor at Lucas Heights in Sydney. Dr Sue Town, physicist Looking in here you’re basically looking at he top of the reactor, 25 uranium fuel elements that we have, various control arms and safety rods that we have, Wilson da Silva Ok, so those things in the middle are basically the fuel rods that drive the reactor? Dr Sue Town Yes… we’ve got 25, they’re Uranium 235 that have been enriched to 60%, the total weight is 280g per fuel element of Uranium 235 plus 238. Wilson da SilvaAnd uranium is what powers most reactors around the world? Whether research reactors or power reactors? Dr Sue Town Right. Basically you have a neutron which bombards an Uranium 235 atom which splits the atom which gives rise to further neutrons coming out of the atom and that then produces fission. Wilson da Silva That’s what causes criticality isn’t it, when you get it to the point where there’s a chain reaction occurring? Dr Sue Town Yes, that’s what a reactor’s all about, basically producing that and being able to control and maintain it … Narration It’s pretty easy really: just pack enough uranium together and a chain reaction occurs. That’s criticality. Now this may be a research reactor, but power reactors work the same way: except that the superhot uranium core turns water instantly into steam, driving turbines and generating electricity — and lots of it. But they do have their drawbacks: they produce tonnes of radioactive waste that stays dangerous for a quarter of a million years. A byproduct is plutonium, which is great for making nuclear weapons. And there’s always the chance, however remote, of a catastrophic meltdown. Wilson da Siva PTC Thorium is also radioactive, although not as much as uranium. No matter how much you pour into the core of a reactor, it can never go critical, or ‘try to blow up’. So what you do is you heat it up. Not with a microwave oven, but with a particle accelerator. Basically a big particle gun which fires neutrons into the core of the thorium reactor — to the point where it is tickling criticality. The only Australian researching thorium reactors is Dr Reza Hashemi-Nezhad of High Energy Physics … We’re going to try to catch a physicist in is natural habitat … (knock, knock). Wilson da Silva So what is this thing going to look like? Dr Reza Hashemi-Nezhad It’s principle is very simple. It’s made of a big container which is 30 metres deep. It contains a coolant vessel inside which is filled with the lead. We have the fuel here, which is made of thorium. And then this beam of the protons is fired through a tube into the middle of the fuel. And you produce a lot of neutrons, and produce … nuclear fission and generate energy.Narration This is one reactor that ain’t ever gonna meltdown. If it tries to overheat, you simply switch off the accelerator … and the reaction just fizzles out. And it produces zero plutonium — so no bombs. The thorium core is so efficient it can even burn old plutonium, as well as nuclear waste, cooking the whole lot into oblivion. Dr Reza Hashemi-Nezhad This sub-critical nuclear reactor is the only logical way of burning the plutonium, producing energy, and getting rid of one of the most dangerous substances on the Earth. Wilson da Silva PTC Thorium reactors do produce some waste, but not much. (points to pile of toilet rolls) If this was the amount of waste produced by a conventional reactor, a thorium reactor would generate about this much. (pull one out, others collapse) Three per cent.The good news is, thorium waste is radioactive for only five hundred years. If you think that’s long, try a quarter of a million. Narration That’s how long conventional waste, on average, stays dangerous. But some of it is radioactive for 20 million years. In a small way, Dr Hashemi-Nezhad is contributing to the design of thorium reactors. He had these samples irradiated at a powerful accelerator in Moscow to try and predict how neutrons might behave in the core of the reactor. Dr Reza Hashemi-Nezhad This is a joint group: couple of teams from Russia, couple of teams from Germany; in Strasbourg, France; and China and India are involved in this project and are doing different bits of work. The final results will be compared with each other. When thorium reactors were first suggested in 1989, scientists just couldn’t believe such a simple idea would work. As often happens in science, the discovery was always there to be made: it just took someone to see the possibility, and pounce on it. Dr Reza Hashemi-Nezhad If you look at it from any angle, it is much safer than existing reactors, and less harmful than even coal-burning power station. Narration There are plans for three reactors in Spain by 2005, while American scientists want to build them to incinerate weapons plutonium. If the science holds true, the first power reactors could be on-line within decades. And there’s enough thorium in the ground to power the planet for another 4,400 centuries. Further Information· Dr Reza Hashemi Nezhad
Room 357
High Energy Physics Department
Physics Building, Physics Road
University of Sydney
· Rochell Buckland
Public Relations
Lucas Heights NSW 2234
Web Links · A Realistic Plutonium Elimination Scheme With Fast Energy Amplifiers And Thorium-Plutonium Fuel, a report produced by the European Organization for Nuclear Research. · Conceptual Design Of A Fast Neutron Operated High Power Energy Amplifier, also produced by the European Organization for Nuclear Research. · Reactors Coupled With Accelerators paper delivered at a From seminar at JRC – ISPRA seminar on July 2, 1996. · Closing the Fuel Cycle with Accelerator Driven Systems, International Workshop on the Physics of Accelerator-Driven Systems for Nuclear Transmutation and Clean Energy, 29th Sept. – 3rd Oct. 1997, Trento, Italy. · Some Safety and Fuel Cycle Considerations in Accelerator Driven Systems NATO Advanced Research Workshop on Advanced Nuclear Systems Consuming Excess Plutonium, Moscow, Russia, 13-16 October 1996. · Further links see –Accelerator Driven Systems and Thorium: an E-Print and Links Library (last updated 13th May 1998). · Homepage of University of Sydney School of Physics
· Background on Carlo Rubbia , the 1984 Nobel Laureate in Physics who has become a key proponent of the ADS system.
Thorium: Is It the Better Nuclear Fuel?
It may turn out to be a quantum leap in the search for economy and safety.
Carlo Rubbia won a Nobel Prize in Physics in 1984 for the discovery of two elusive high energy particles, called the W and the Z. The discovery was a feat not only of physics, but of engineering. He is good at both, and now has another idea which could revolutionize the methods we use to retrieve nuclear energy.
You may never have heard of thorium. It is a plentiful element; there is more of it in the earth’s crust than uranium. No, it is not fissionable. But it can be made into a low weight isotope of uranium that is fissionable. Rubbia thinks it may be worth the trouble to do that, even if it is a roundabout route to nuclear fission. countries.
A good introduction to Rubbia’s idea is in “
Megawatts and Megatons,” (pp153-163) by Richard Garwin and Georges Charpak, Knopf, NY 2001 (originally published in 1997 in French). Another summary, just 3 pages long, is in the CERN Courier, a publication of the European collider laboratory, of April 1995, available on the web at . The CERN report closes with this sentence: “With the heavy ecological implications of present nuclear and conventional energy sources, it is surprising how little R&D work is being invested anywhere in this potentially rewarding alternative energy solution.”
What is special about thorium?
(1) Weapons-grade fissionable material (uranium233) is harder to retrieve safely and clandestinely from the thorium reactor than plutonium is from the uranium breeder reactor.
(2) Thorium produces 10 to 10,000 times less long-lived radioactive waste than uranium or plutonium reactors.
(3) Thorium comes out of the ground as a 100% pure, usable isotope, which does not require enrichment, whereas natural uranium contains only 0.7% fissionable U235.
(4) Because thorium does not sustain chain reaction, fission stops by default if we stop priming it, and a runaway chain reaction accident is improbable.
Besides, the priming process is extremely efficient: the nuclear process puts out 60 times the energy required to keep it primed. Because of this, the device is also called, (quite inappropriately) an “Energy Amplifier.”
Naturally occurring thorium is in the form of the stable isotope,
90Th232. Notice that thorium is just two places removed on the periodic table from Uranium. In a sequence of nuclear processes exactly like those by which the non-fissionable isotope, 92U238 is bumped up through Neptunium to Plutonium, 94Pu239, Thorium can be bumped up to a light weight isotope of Uranium, 92U233. (See p 135, Eq 15.01 and 15.02 of “A serious but not ponderous book about Nuclear Energy“.) In each case, a non-fissionable isotope is converted to a fissionable one.
Plutonium, while highly radioactive, can be shielded and concealed for shipping and storage, because the alpha rays that it emits do not penetrate lead. On the other hand, uranium233, the weapons-grade material that could be recovered from the thorium reactor, can not be as easily concealed. U233
is almost inextricably accompanied by 0.1% of U232, which, after a series of dissociations (to thallium208) emits gamma rays that penetrate everything. Here is the thorium sequence in the Rubbia reactor: A neutron is captured by90Th232, which makes it 90Th233. 90Th232 + 0n1 -> 90Th233 [1]Thorium-233 spontaneously emits a beta particle (an electron from the nucleus, see p 173), leaving behind one additional proton, and one fewer neutron. (“…Nuclear Energy” p134) This is called “beta decay.”
90Th233 -> 91Pa233 + ß [2]
The element with 91 protons is Protactinium (Pa). The isotope 91PA233 also undergoes beta decay,
91Pa233 -> 92U233 + ß [3]
The U233 isotope that is produced in step [3] is fissionable, but has fewer neutrons than its heavier cousin, Uranium-235, and its fission releases only 2 neutrons, not 3.
92U233 + 0n1 -> fission fragments + 20n1 [4]
If this sequence [1 through 4] is to replicate itself, it would require one neutron to generate the next U233 nucleus [1–3] and another would be required to induce the U233nucleus to fission [4]. A chain reaction, then, could occur only with 100% utilization of the 2 neutrons emitted in [4]. 100% utilization means none can be allowed to get away, an ideal that can not occur in practice. With 98% utilization, the generation ratio (p 87-93) would be 0.98, and the half-life of the decline of the number of fissions per generation would be 50 generations. (1000 fissions in the zeroth generation would decline to 1000/e, or 368, fissions in 50 generations.)
This means that by itself, the fission process would die out very quickly. With a steady supply of “priming” neutrons, one can obtain, on the average, 50 new fissions from each priming neutron. There is, of course, a cost in providing the priming neutrons. But because the energy cost of the priming neutron is about 30 to 60 times less than the energy yield of the fissions it triggers, there is a net gain of energy of about 30 to 60. This is why it is called an Energy Amplifier (EA).
The priming neutrons are emitted in a process called “spallation,” which is the induced splitting of an otherwise non-fissionable large nucleus. In the EA, a proton beam impinges on lead, the high energy protons splitting lead nuclei, leading to release of neutrons. In Rubbia’s design, the molten lead doubles also as primary coolant. The diagram at the left shows the proposed arrangement, most of it below ground level. High energy protons emerge through a window in the tip of the proton beam tube inside the core. Protons split lead nuclei, with neutrons emitted into the core. The molten lead carries nuclear heat upward by convection.
Pumping is required only in the secondary coolant loop, which carries the heat to where steam is made for the turbines. All other circulation is convection-driven, with no moving machinery. The lead and air circulation is guided along partitions that are not shown.
The lead vessel is nearly 30 meters long and 6 meters in diameter, and contains 10,000 tons of lead. Control rods are not needed, either to regulate energy production or to stop fission in an emergency, because the fission rate is determined by the proton accelerator. If the accelerator stops sending protons, fission stops almost instantly. In an emergency, the proton accelerator can be switched off by a trigger signal, or it can be shut off automatically if overheating causes the expanding lead to overflow into the accelerator.
Once fission is stopped, there is still the heat released from radioactive fission fragments that were produced before the shut-down. Although this rate of heat generation is a small fraction of that during normal operation of the reactor, the after-shutdown heat can accumulate rapidly if it is not removed (p 208). In the conventional uranium reactor this heat can be sufficient to melt the core and the bottom of the containment.
There is no reason to believe that the prevalence of short-lived radioactive fission fragments (after fission is stopped) will be much different in the Rubbia reactor from that in uranium 235 reactors (p 208). But the EA is undoubtedly a safer reservoir for the after-shutdown heat than the conventional reactor, because it is filled with heat absorbing material (lead) that does not leak, does not require pumping to distribute the heat evenly, and will not boil away or make bubbles, as water does. Simple calculations suggest that the lead in this reactor has sufficient heat capacity to keep the temperature in the reactor below 1300oC even in the worst case, if the cooling system shuts down completely and no heat is removed from the reactor.
The radioactive waste from the thorium reactor contains vastly less long-lived radioactive material than that from conventional reactors. In particular, plutonium is completely absent absent from the thorium reactor’s waste. While the radioactivity during the first few days is likely to be similar to that in conventional reactors, there is at least a ten-fold reduction of radioactivity in the waste products after 100 years, and a 10,000 fold reduction after 500 years. From a waste storage point of view, this is a significant advantage.
It is certainly premature to celebrate this technology yet. Much of the feasibility data is from small scale tests and from simulations. There are technical challenges that will have to be overcome. One of these is to find a containment material that does not have the nasty tendency that steel has to dissolve in molten lead.
An encouraging fact is that so far, the simulations and tests have supported the theoretical predictions, which is a testament to the engineering savvy of Carlo Rubbia. In addition to the CERN group, several laboratories in the US, Japan, and Russia are working on various aspects of the EA technology.

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